In recent years, research on ad-hoc networks in which wireless terminals are autonomously connected to each other has been developed. In an ad-hoc network, an access point is not provided, and each wireless terminal has routing information. Each wireless terminal forwards a packet received from an adjacent terminal to another adjacent terminal according to the routing information. Such a configuration allows a packet to be transmitted to a desired destination within the ad-hoc network.
In the ad-hoc network, a network environment frequently changes with, for example, a change in a radio field intensity or moving of a wireless terminal. Accordingly, each wireless terminal connected to the ad-hoc network performs routing control before the wireless terminal communicates with another wireless terminal. As an example, a wireless terminal newly connected to the ad-hoc network broadcasts presence information to an adjacent wireless terminal. The wireless terminal that has received the presence information generates routing information corresponding to the presence information and further transmits this routing information to an adjacent wireless terminal. As a result, wireless terminals in the network are notified of the routing information related to the new wireless terminal.
As a related art, the following route information relay method has been proposed. Each node connected to an ad-hoc network receives routing information from adjacent nodes. Each node acquires quality information indicating the quality of routes indicated by the route information with respect to the received route information. Each node determines whether or not a plurality of pieces of route information in which nodes that are the destinations of packets are identical are received for each predetermined period with respect to the received route information. As a result of the determination, if the plurality of pieces of route information in which the nodes that are the destinations are identical have been received, each node compares quality information for each of the plurality of pieces of route information received and selects route information corresponding to the quality information indicated to be the highest quality as a result of the comparison. Each node relays the selected route information to the adjacent nodes. (See, for example, Japanese Laid-open Patent Publication No. 2009-267532.)
For conventional ad-hoc networks, however, measures are insufficient for problems caused by hidden terminals. In the following, problems caused by hidden terminals will be described with reference to FIG. 1.
A node GW and nodes A-E are connected to an ad-hoc network illustrated in FIG. 1. Assume that the node A can perform favorable communication with the node GW. Although the nodes B and C can receive radio waves from the node GW, the quality of their communication with the node GW is not favorable. Radio waves from the node GW do not reach the nodes D and E. The node GW collects data from the nodes A-E.
In the ad-hoc network above, the node B holds the following routing information.
“When a destination of a data packet is node GW, this data packet is transmitted or forwarded to node A.”
In this case, when the node B generates a data packet addressed to the node GW, the node B transmits this data packet to the node A. By so doing, the node A forwards the data packet received from the node B to the node GW. As a result, the node GW receives data of the node B. In this case, the node GW returns to the node A an ACK packet corresponding to the received data packet. Hereinafter, assume that the node B transmits data packets to the node GW via similar routes if necessary. In this case, the node A receives the data packet from the node B and receives the ACK packet from the node GW. However, the node GW and the node B can communicate with each other. Accordingly, the node GW and the node B can utilize, for example, CSMA/CA (Carrier Sense Multiple Access Collision Avoidance) to control their transmission timings. As a result, a packet collision which would occur at the node A is avoided or suppressed.
The node D holds the following routing information. “When a destination of data packet is node GW, this data packet is transmitted or forwarded to node A.”
In this case, as with the node B, the node D transmits the data packet to the node A. By so doing, the node A forwards the received data packet to the node GW, and the node GW returns an ACK packet corresponding to the received data packet to the node A. In this case, the node A receives the data packet from the node D and receives the ACK packet from the node GW.
However, radio waves from the node GW do not reach the node D. And radio waves from the node D do not reach the node GW. That is, the node D is a “hidden terminal” for the node GW, and the node GW is a “hidden terminal” for the node D. Thus, the node GW and the node Dare incapable of performing transmission timing control to avoid a packet collision. As a result, when the node D transmits to the node A a data packet addressed to the node GW, a packet collision may occur at the node A.
The node D may relay a data packet transmitted from another node (e.g., the node E) and addressed to the node GW. In this case, the node D forwards the data packet transmitted from the node E to the node A, and the node A forwards this data packet to the node GW. And the node GW returns corresponding ACK packet to the node A. Accordingly, also in this case, a packet collision may occur at the node A.
As described above, when each node (a wireless terminal or a wireless communication device) forwards a packet by simply following routing information without considering a hidden terminal, the occurrence frequency of packet collisions increases. An increased occurrence frequency of packet collisions decreases a communication speed, causing a risk of congestion. That is, in the prior art, measures are insufficient for problems caused by hidden terminals, so communication efficiency decreases in some situations.